Automatic gain control (AGC) is often used with amplification circuitry to control the amplitude of amplified signals. In radio receivers, for example, communication signals received are amplified for further signal processing. These received signals may be of varying signal strength. If amplified without automatic gain control, the resultant signals may have too much or too little gain, with a corresponding fluctuation of signal amplitudes. Variations in signal amplitude may cause significant problems in circuitry attempting to recover information present in the received signals. Accordingly, an AGC circuit is usually employed in an effort to present a signal with relatively constant amplitude.
A typical AGC circuit detects amplitude changes in a signal to be amplified and controls the gain of the amplification stage such that if the input signal strength diminishes, the amplification gain is increased, and if the input signal strength increases, the gain of the amplification stage decreases. Thus, the automatic gain controller attempts to control the output of the amplification stage such that a signal has a relatively constant amplitude output. This type of AGC is common in communication circuitry.
AGC circuits are often implemented using analog signal processing techniques. Generally, a feedback system is used in which the amplitude or energy level of the output signal from the amplifier is compared against a desired reference value, such as a reference voltage. Depending on the comparison results, a feedback signal is coupled to the amplifier to vary its gain. Circuitry in the feedback system controls the attack time and release time of the AGC. For example, a resistor capacitor combination is typically used to provide a fixed time constant for the release, where the selected value of the capacitor represents a trade off between the lowest frequency and the response time required. Automatic level controllers, limiters, and compressors have construction based on the same principles.
A major problem with analog automatic gain controllers or level controllers is that they suffer from a trade off between release time and the lowest frequency of operation. Analog gain loops also suffer from instability and settling time issues. Digital control AGC circuits avoid many issues associated with analog circuits, such as by using counters and time out timers to control attack and release time. However, prior art digital AGC circuits usually involve microprocessors or digital signal processors that add substantial cost and complexity to a design, and require increase power consumption over analog implementations.
It is desirable to have an improved apparatus and method for providing automatic gain control, automatic level control, that offer the flexibility in attack and release time of digital circuits, while offering the relative simplicity of analog circuits. Therefore, a new method and apparatus for automatic level or gain control is needed.